1. Field of the Invention
The subject invention relates to solar photovoltaic cells and, more specifically, to method for manufacturing low cost metallization layers for such cells and the resulting cell device structure.
2. Related Art
The silkscreen, silver-paste metallization technology has been developed mainly by the “traditional” diffused junction solar cell manufacturers. In such cells, a top layer of silicon nitride is used and then the metallization is formed on top of the silicon nitride. However, electrical contact must be made between the metallization and the emitter—through the silicon nitride layer. Therefore, silkscreen is used to deposit the silver paste and then the cell is annealed at high temperature (e.g., 950° C.) so that the silver paste diffuses through the silicon nitride layer and makes contact to the emitter. Since diffused-junction solar cells make the bulk of the solar market (by a very large margin), silver-paste technology became a de facto standard in the solar cell industry and much of the manufacturing and development efforts are directed at improving the conductivity of the resulting metallization using silver paste.
A specific metallization layer that is particularly relevant to the subject invention is busbar and fingers over a transparent conductive oxide (TCO). One solar cell architecture that incorporates a silver busbar and fingers over TCO is known as the HIT cell, available from Sanyo® of Japan. FIG. 1 illustrates the general structure of the HIT module. A high quality (Czochralski grown) single crystal silicon wafer of n-type is used as the substrate 100. The substrate 100 is about 200 micron thick and a square of about 125 mm by 125 mm. The substrate surfaces are texturized to form pyramid shapes throughout the surface, but this is not shown in FIG. 1 due to the minute size of the pyramids. The top surface is coated with a thin layer of an amorphous intrinsic silicon layer 105. A thin layer 110 of amorphous p-type silicon is deposited over the intrinsic layer 105. A layer of TCO 115, e.g., ZnO2, ITO (Indium Tin Oxide), or InSnO, is deposited over the p-type layer. Then, busbar 120 and fingers 125 are fabricated over the TCO, generally by silk screen followed with anneal. The same is done on the bottom surface, wherein an i-layer 130, p-layer 135, and TCO layer 140 are deposited on the bottom surface, followed by busbar 145 and fingers (not visible in FIG. 1). The HIT cell, while being of relatively high efficiency (currently available at about 20% efficiency) is very expensive to manufacture. While part of the cost being the high grade silicon substrate used, other part of the cost is the high cost of the silver paste-based busbar and fingers. Additionally, the silver paste-based busbar and fingers pose a reliability problem in that they tend to delaminate with time.
FIG. 2 illustrate another structure, known as SmartSilicon®, and available from Sunpreme of Sunnyvale, Calif. A rather “dirty” metallurgical grade silicon (MG silicon) is used for fabricating substrate 200, using casting and solidification technique. Metallurgical grade silicon is generally of 3-5 “nines” purity, meaning 99.9%-99.999% pure, compared to Czochralski grown substrates, which are of nine-nines purity and even higher. Metallurgical grade silicon is generally used in the manufacture of aluminium-silicon alloys to produce cast parts, mainly for the automotive industry. It is also added to molten cast iron as ferrosilicon or silicocalcium alloys to improve its performance in casting thin sections, and to prevent the formation of cementite at the surface. MG silicon has been thought to be useless for semiconductor and solar applications. See, e.g., Towards Solar Grade Silicon: Challenges and Benefits for Low Cost Photovoltaics, Sergio Pazzini, Solar Energy Materials & Solar Cells, 94 (2010) 1528-1533 (“As shown before, MG grade silicon is much too dirty to be employed for EG and PV applications.”), and Solar Energy website of the U.S. Department of Energy: “to be useful as a semiconductor material in solar cells, silicon must be refined to a purity of 99.9999%.” (Available at http://www1.eere.energy.gov/solar/silicon.html.) This is generally referred to as 6N, or solar grade silicon, SoG Si. Therefore, efforts have been made to produce what is referred to as “upgraded” metallurgical silicon (UMG silicon). However, to date, these efforts have not shown great success and come at high cost, especially for the high energy required for the “upgrade” process. Conversely, Sunpreme has shown that by using p-type doped MG silicon substrate and forming specific layers of amorphous silicon, a relatively cheap solar cell can be formed that has conversion efficiency higher than that of conventional thin-film solar cells.
The SmartSilicon solar cell is formed using a p-type MG silicon substrate 200, forming an amorphous intrinsic layer 205 on the top surface, forming an amorphous n-type layer 210 over the intrinsic layer 205, forming a TCO 215 over the n-layer. A back metallization layer is formed by depositing a titanium layer 230 over the entire back surface of the substrate 200, and depositing a layer of aluminum 235 over the titanium layer 230. The busbar 220 and fingers 225 are formed of silver, using the silk screen method. The SmartSilicon cell's attractiveness is in its conversion efficiency being competitive with that of pure silicon solar cells, while using an extremely cheap MG silicon substrate. Consequently, the relative cost contribution of the silver metallization process to the cost of the entire cell increases.
While the solar industry embraces the silkscreen silver paste metallization process, the subject inventors have determined that there is a need to provide a cost-effective solution for busbar and finger metallization over a TCO, that is cheaper and more reliable than silkscreen silver paste, and that has lower resistivity than silver paste.